Graphite is a crystalline form of carbon comprising atoms bonded in flat planes, with weaker bonds between the planes. A useful form of graphite is a flexible sheet or foil. Because of graphite's stability at high temperatures, flexible graphite foil is useful in high temperature applications as, for example, gaskets and valve packings, and is often used as a replacement for asbestos gasket materials.
Flexible graphite is made by first treating graphite flakes with substances that intercalate into the crystal structure of the graphite and react to form a compound of graphite and the intercalant. Upon heating at a high temperature, the intercalants in the graphite crystal form a gas, which causes the layers of the graphite to separate, and the graphite flakes to expand or exfoliate in an accordion-like fashion in the c-direction, i.e. the direction perpendicular to the crystalline planes of the graphite. The exfoliated graphite flakes are vermiform in appearance, and are therefore commonly referred to as worms. The worms are then compressed together into sheets. Unlike the original graphite flakes, the sheets are flexible and can be formed and cut into various shapes.
A common method for manufacturing flexible graphite foil is described by Shane et al. in U.S. Pat. No. 3,404,061. In the typical practice of the Shane et al. method, natural graphite flakes are intercalated by dispersing the flakes in a large excess of a solution containing an oxidizing agent, such as a solution of sulfuric acid and nitric acid, after which the excess solution is drained from the flakes. The quantity of intercalation solution retained on the flakes after draining is typically greater than 100 parts of solution by weight per 100 parts by weight of graphite flakes (pph), more typically about 150 to 200 pph.
Of the intercalation solution retained on the flakes after draining, only a fraction actually reacts with the graphite. This excess portion of the solution is generally removed by washing the flakes with water. This eases handling of the intercalated flakes and helps prevent degradation of the graphite from excess oxidant in the solution.
After washing with water, the intercalated graphite flakes are dried and then exfoliated by exposing them to a flame for a few seconds at temperatures greater than 700.degree. C., more typically 1000.degree. C. or higher. The exfoliated graphite flakes or worms are then compressed and rolled into flexible graphite sheets.
Hirschvogel et al. in U.S. Pat. No. 4,091,083 disclose a method for forming intercalated graphite which comprises dispersing graphite particles in sulfuric acid, and adding hydrogen peroxide while maintaining the graphite particles in a dispersed state. 100 to 200 parts of sulfuric acid/hydrogen peroxide solution are used for 100 parts of graphite flakes. After graphite is intercalated the graphite is separated from the solution and washed to remove excess residual acid.
A problem with these prior art methods is that they inefficiently utilize the intercalation solutions. At least four times as much solution is retained on the graphite flakes as reacts with the graphite to form an intercalated graphite compound. The excess solution remains on the surface of the flakes, and must be removed by washing the flakes with water. This causes a significant waste disposal problem, since the acid washed from the intercalated graphite must be neutralized before disposition in the sewage system. The neutralization step is costly. Therefore, a method avoiding production of an acid waste wash solution would be desirable.
An additional environmental problem occurs when phosphate compounds are used in the intercalation solution, as disclosed, for example, in United States patents 4,146,401 and 4,400,433. In these methods, phosphate compounds are used to impart oxidation resistance to compressed exfoliated graphite foil, by intercalating graphite flakes with phosphates in addition to other intercalating agents. When the graphite flakes are washed after intercalation, approximately 80% of the phosphate retained on the flakes is unreacted and must be washed off and disposed of. Phosphate ions are environmental contaminants, and their discharge into the environment is restricted. It would therefore be desirable to produce an oxidation resistant graphite product which minimizes the production of phosphate containing waste water.
Another problem associated with prior art methods is that the volume to which intercalated graphite flakes exfoliate decreases with the age of the intercalated graphite, even when the flakes are washed and dried. This occurs if the flakes have a high ash content (greater than about 1 wt. %) and have been intercalated by a process which leaves residual oxidant on the flakes, as, for example, with the nitric acid/sulfuric acid system. This aging of the intercalated flakes requires adjustments in the manufacture of flexible graphite and generally leads to decreased uniformity in area/weight of the flexible graphite foil. The age-deterioration of exfoliated flake volume also is detrimental to the strength properties of the graphite foil, and eventually the exfoliated flake volume may deteriorate to a point that acceptable graphite foil cannot be produced. It would therefore be desirable to produce intercalated graphite by a method which minimizes the problem of deterioration due to aging.